A. Field of the Invention
This invention is in the field of storage tanks for liquid products, and particularly liquid petroleum storage tanks each having a floating roof and a drainage system to drain excess accumulation of water on top of the floating roof.
B. Background of the Invention
Certain storage tanks for oil and other liquid petroleum products are provided with a floating roof or double deck roof which covers and protects the liquid product, reduces the escape of harmful vapors, and moves vertically on the surface of the liquid product as its volume changes. A typical floating roof for covering a liquid product in a storage tank includes a deck part whose circumference substantially conforms to the horizontal cross-sectional shape of the storage tank and is provided with one or more seals extending between the outer periphery of the floating roof deck part and the inner walls of the tank. The roof floats on top of the stored liquid product and rises and falls as the amount of the liquid product increases and decreases. It is known to provide a drain pipe extending downward through the top surface of deck, to drain water accumulated on the top of the roof and to provide a passage or conduit for the drained water to flow either into the liquid petroleum in the tank or via a discharge duct downward through and exiting said storage tank.
A problem that occurs with these rain water drain pipes is escape upward through said pipes of fumes from the liquid petroleum in the tank. Even where the drain pipe has a closure valve, fumes can escape if the valve is open without water descending or while the valve is temporarily open to allow water to descend.
Various approaches have been proposed for draining water from storage tank floating roofs. For example, U.S. Pat. No. 2,560,586 to Michaels discloses a floating roof drain which drains water collected over a valve of the floating roof which closes and opens a drain passing. The weight of the water collected over the valve pushes down a cover against the buoyancy force of a float connected to the valve to allow the water collected to flow from the roof. The valve is again closed when the depth of the water on the roof decreases until it is no longer sufficient to hold the valve open against the buoyancy force of the float.
U.S. Pat. No. 2,913,138 to Swick describes floating covers for tanks in which a drainage device is located at a low point of the roof structure in a deck. The drainage device comprises a cylindrical sump, a sump bottom formed with a shallow depression which constitutes a downward flow passage, a cylindrical neck extending from the bottom of the sump and an annular float member loosely surrounding the sidewall of the neck. A mercury seal is provided on the bottom and the float member rests on the bottom of the pool of mercury. Accumulated water in the sump buoys up the float member to interrupt the mercury seal and thereby provide a conduit for water to drain across the surface of the pool.
U.S. Pat. No. 3,883,032 to Fisher discloses an automatic drain valve for a floating roof which includes an orifice and a larger disk located under the orifice. A float attached to the disk allows the valve to be biased closed and water gathered on top of the roof will open the valve which permits the water to drain through the roof into the tank beneath to join the body of liquid product with the tank.
Japanese Patent No. JP5077883 to Kunio discloses in FIG. 1 an emergency drain device for a floating roof in which a drain pipe 8 runs from accumulated water 9 atop the floating roof 1 down through drain pipe 8 into stored liquid 2 in the storage tank. As seen in FIG. 2 float 12 which floats on accumulated liquid 9 on the deck and is at the upper end of the drain pipe 8, prevents vapor from flowing out of the drain pipe. A weight-attached guide member 13 makes the float return down to the upper end of the drain pipe. With this apparatus there remains the possibility for some vapor to escape under a variety of circumstances.
The above systems for draining water from floating roofs do not overcome the problem of fumes escaping upward while a valve is open to allow accumulated rain water to flow downward through the valve.
It is, therefore, an object of the present invention to provide an apparatus for effectively and reliably draining accumulated rain water from a floating roof on a liquid product tank by a valve mechanism that bars or substantially reduces the escape of petroleum gas fumes when the water drain valve is open.
An additional object is to provide for a floating roof an emergency drainage system which will automatically open under predetermined conditions and automatically bar escape upward of fumes through said valve.
A still further object is to provide an uncomplicated but reliable emergency drainage system which will be normally closed and will automatically open merely from the accumulation of a predetermined head of water on the roof in the vicinity of the emerging drainage valve.
A further object of the invention is to provide an automatic roof drain apparatus that is of simple and rugged construction and inexpensive to manufacture and install.